For crystal growth of a GaN group material, a substrate free of lattice match, such as sapphire, SiC, Spinel, recently Si and the like, is used, because there is no substrate that lattice matches with GaN group materials. However, a manufactured GaN film contains dislocations amounting to 1010 dislocations/cm2 due to the absence of lattice match. While high luminance light emitting diodes, semiconductor lasers and the like have been realized in recent years, reduction of a dislocation density is desired for achieving improved properties.
To avoid defects, such as dislocation and the like, caused by different lattice constant and the like, the same crystal as the material to be crystal grown is used. For example, for crystal growth of a GaN group semiconductor, a GaN substrate is used. However, a large-sized product has not been obtained, and therefore, sapphire and the like are actually used as a substrate. There has been proposed a method in recent years, wherein, for vapor phase growth on a GaN base layer grown on sapphire, the aforementioned base layer is partially masked to effect a selective growth for crystal growth in a lateral direction, whereby a high quality crystal having a reduced dislocation density is obtained (e.g., JP-A-10-312971). By growing this film thick and separating and removing the substrate, a GaN crystal can be obtained. Due to the problem of occurrence of cracks and breakage of substrate resulting from differences in lattice constant and in thermal expansion coefficient, a substrate having a large area has not been obtained.
The above-mentioned JP-A-10-312971 discloses a method for obtaining a film having a reduced dislocation density. However, a new problem has been found that, in the part grown in a lateral direction on a mask layer, the C axis tilts in a slight amount toward the direction of lateral growth, and degrades the crystal quality (Abstracts of MRS 1998 Fall Meeting G3.1). This can be confirmed through measurement (¢ scan) of the incident orientation dependency of X-ray rocking curve measurement (XRC). That is, a full width at half-maximum (FWHM) of X-ray rocking curve by incident X-ray from the direction of lateral growth is greater than the FWHM value by X-ray from a stripe direction of a mask layer, which means the presence of orientation dependency of the micro tilting of the C axis. This suggests a possibility of inducing a number of new defects in the junction part of the lateral growth on the mask.
As the mask-layer material, SiO2 is generally used. A problem has been clarified that, when a layer is laminated thereon by crystal growth, the Si component transfers into the crystal growth layer, constituting a problem of autodoping contamination.
When a semiconductor material containing Al, such as AlGaN, is grown on a substrate containing an SiO2 mask layer, crystal growth occurs on the mask layer, too, preventing effective selective growth itself.
In an attempt to solve such a problem, a method has been proposed wherein a stripe groove processing is applied to a substrate having a buffer layer and a GaN layer formed on an SiC base substrate, which groove reaches the SiC layer to form a convex, and crystal growth is done from the GaN layer on the top of this convex (Abstracts of MRS 1998 Fall Meeting G3.38). According to this method, a selective growth without an SiO2 mask layer is possible, whereby various problems caused by the use of the aforementioned SiO2 mask can be overcome.
For the above-mentioned method, a sapphire substrate can be used as the base substrate and the method thereof has been disclosed (e.g., JP-A-11-191659). The above-mentioned method requires steps of crystal growing a buffer layer material and a GaN group material on a sapphire base substrate, taking the substrate out from a furnace for growth, applying a groove processing, and crystal growing again, thus posing a new inconvenience of complicated production process, increased number of steps, higher cost and the like.
An attempt has been also disclosed in the (Abstracts of the Japan Society of Applied Physics 99 autumn 2P-W-8) wherein a step is made on a GaN substrate and embedding growth is performed to secure a region having a low dislocation density. In this case, a low dislocation density region is formed in a part of the embedded layer.
To obtain a low dislocation density region by the above-mentioned method, however, the intervals between convex parts need to be widened or the depth of concave part needs to be increased. To this end, the layer needs to be grown thick by embedding for a long time, which in turn intrinsically poses various problems of occurrence of cracks due to the thick film forming, costly manufacture due to the long time spent, and the like.
There is also an attempt to crystal grow a GaN group material on an Si substrate. However, growth of a GaN group crystal results in cambers and cracks caused by difference in the thermal expansion coefficient, thereby preventing crystal growth having a high quality.
It is therefore an object of the present invention to avoid various problems caused by ELO growth using a conventional mask layer and achieve simplification of the production process, in view of the above-mentioned problems. The present invention also aims at solving the problems caused by the embedding growth of a step structure without a mask. In addition, the present invention aims at solving the problem of selective growth of AlGaN, which has been conventionally unattainable. The present invention further aims at suppressing the occurrence of cambers and cracks associated with the use of an Si substrate and the like.
In view of the above-mentioned problems, it is an object of the present invention to provide a GaN crystal having a large area. The present invention also aims at avoiding various problems caused by the ELO growth using a conventional mask layer and simplifying the production process.